Method of reducing amplitude noise of solid lasers with resonator-internal frequency doubling, and an arrangement for carrying out this method

ABSTRACT

The invention concerns a method of reducing the amplitude noise of solid lasers (e.g. Nd-YAG, Nd:YAlO, Nd:YLF, Nd:YVO 3 , etc.) with resonator-internal frequency doubling (for example by KTP, LBO, BBO, KNbO3, LiNbO 3 , etc.). According to the invention, some of the frequency-doubled ejected laser radiation or the fundamental wave radiation is guided onto a photodetector (in particular a photodiode) whose electrical output signal undergoes bandpass filtering which filters out a frequency range typical of the noise. This output signal, optionally after rectification, quadrature and/or integration, is then subjected to temperature regulation or control which corrects the temperature of the frequency-doubling crystal such that the output signal is minimized after the bandpass filter and thus the amplitude noise of the laser radiation is minimized.

FIELD OF THE INVENTION

The present invention pertains to a process and device for minimizingthe amplitude noise of solid lasers with frequency doubling within acavity and more particularly to a process and device wherein a solidlaser material and a frequency doubler crystal are located in a commoncavity.

BACKGROUND OF THE INVENTION

Solid lasers (using mostly rare earth-doped crystals or glasses, e.g.,Nd:YAG, Nd:YV0₄, Nd:YAlO, Nd:YLF, Nd:glass or other, similar solidmaterials) with frequency doubling within the cavity have been known fora long time and are used for many applications in laser technology. Whatis used here is the generation of the second or higher harmonicvibrations in materials (mostly crystals, which have no inversioncenter, e.g., KTP, LBO, BBO, KNbO₃, LiNbO₃ or other) with a highnonlinear coefficient, which generates light of double (or multiple)frequency of the irradiated light wave by anharmonic vibrations of thelattice atoms, excited by an incident light wave. The process ofgenerating higher harmonics strongly depends on the power density (cf.,e.g., Kochner, Solid-State Laser Engineering), so that to generatefrequency-doubled laser radiation of high efficiency, the nonlinearcrystal is often introduced (at least in the case of continuouslyworking (cw) lasers) either into the cavity of the laser itself or intoa separate cavity (see above or, e.g., Yariv, Quantum Electronics, 3rded., p. 402). Even though the latter case of a separate cavity for thefrequency doubler offers the fundamental advantage of low amplitudevariations, this arrangement is characterized by a considerablecomplication due to the fact that the cavity belonging to the frequencydoubler crystal must be actively stabilized to the frequency of thelaser cavity and the laser radiation should be possibly asingle-frequency radiation to achieve a high efficiency. The first caseof introducing the frequency doubler crystal into the laser cavity issubstantially less complicated compared with this; it is possible towork in this case with lasers which emit in longitudinal modes rangingin number from a few to many; the cavity mirrors are usually selected tobe highly reflective mirrors for the laser wavelength in order toachieve a maximum increase in power in the cavity and thus the highestpossible efficiency of doubling; at the same time, the output mirror ishighly transmittent for the frequency-doubled radiation in order to beable to properly decouple it from the cavity.

However, this arrangement has a loud amplitude noise, which is aninherent feature of the system and which was first described, to thebest of our knowledge, by T. Baer in J. Opt. Soc. Am. B, Vol. 3, No. 9,September 1986, p. 1175. There are many different approaches to explainthis noise. Baer explains this by a competition of different modes(since the actually most intense mode is doubled best, it is attenuatedmost by the decoupling from the laser cavity, and another longitudinalmode will now become the most intense one, etc.). Other explanations arebased on the total frequency generation or on the competition betweenmodes of different polarization (cf., e.g., EP 0 457 590 A2). However,all these mechanisms are probably involved in the noise process at thesame time.

The fact that the laser may have a very low-frequency noise, which ismanifested by a "flickering" of the laser beam, whose degree ofmodulation may reach up to 100%, is especially disturbing for manyapplications. This noise is highly chaotic because of the nonlinearrelationship of the doubling efficiency (see Koechner, see above);stable states may become temporarily established, which may be abruptlyfollowed by loud noise. This phenomenon has been investigated in theliterature in detail (see, e.g., Phys. Rev. A, Vol. 41, No. 5, March1990, p. 2778, or Opt. Comm., Vol. 118 (1995), p. 289). Preliminarycontrol models which are to eliminate this chaotic noise have also beendesigned, but the control bandwidth of these controllers is currentlytoo narrow by several orders of magnitude (bandwidths markedly exceeding1 MHz were necessary for a nonlinear control loop) in order topractically suppress the noise (see, e.g., Phys. Rev. A, Vol. 47, No. 4,April 1993, p. 3276).

Other approaches to minimizing the noise are based according to thestate of the art on the introduction of a quarter-wave plate (see U.S.Pat. No. 4,618,957) or a Brewster plate (see DE 3 917 902 A1) into thecavity or on the temperature stabilization of the doubler crystal (seeEP 0 329 442 A2). However, all these solutions according to the state ofthe art have substantial drawbacks.

Even though the introduction of an additional element into the cavity(quarter-wave plate or Brewster plate) makes it possible to extensivelysuppress the flickering of the laser, such elements in the cavity are tobe adjusted very accurately, which increases the expense of manufacture,and these elements always lead to higher losses in the cavity (becauseof residual reflections and scattering) even in the case of the bestadjustment, so that these higher losses also drastically reduce thepower density and consequently the efficiency of doubling.

In contrast, the stabilization of the temperature of the doubler crystal(EP 0 329 442 A2) can be accomplished without such additional elementsand it also makes it possible to markedly reduce the laser noise atequal power density. The reason for this might be that in the case ofthe angle-dependent phase matching (and such phase matching is involvedhere, cf. Koechner, p. 528), the doubler crystal itself acts as aquarter-wave plate of a very high order, which has the same effect as anadditionally introduced quarter-wave plate, but without having to beadditionally adjusted or without offering additional reflection orscattering surfaces. However, the exact adjustment of the length of thedoubler crystal to an integer multiple of λ/4 takes place by an exactadjustment of the length of the doubler crystal via the temperature.

This process already comes very close to the process according to thepresent invention, but it still has an essential shortcoming. Since thelaser cavity changes its temperature in the course of the operation,especially during changes in the environmental conditions or if athermal equilibrium is reached only incompletely, there will becontinuous changes in the length of the laser cavity and also in theexact temperature of the doubler crystal. Therefore, a purestabilization of the crystal length to the temperature permits onlyoperation in a very narrow temperature window under definitely constantenvironmental conditions and at the stable, steady thermal equilibrium(see EP 0 329 442 A2). However, these conditions are not usuallysatisfied in a laser operated under the real conditions of anapplication.

SUMMARY AND OBJECTS OF THE INVENTION

The primary object of the present invention is therefore to develop aprocess as well as an arrangement for carrying out the process, whichmakes it possible to actively adjust the optimal doubler lasertemperature associated with the lowest laser noise (flickering) tochanging environmental conditions or other disturbing effects and thusto make possible the practical use of the laser even outside definedenvironmental conditions and independently from the reaching of athermal state of equilibrium.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of the control process according to thepresent invention, the elements drawn in broken lines being optional;

FIG. 2 is a graph of the typical noise of an in-cavity frequency-doubledlaser with an abrupt drop in output (flickering) (time axis: 10msec/unit);

FIG. 3 is a graph of the noise measurement of an in-cavityfrequency-doubled laser in the state of intense flickering (top curve)and in the stable state without flickering (bottom curve); and

FIG. 4 is a graph of the noise measurement of a laser stabilizedaccording to the present invention (time axis: 10 msec/unit).

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, the laser beam 7 is generated in a mirrorcavity 1 with a laser medium 15 and a frequency doubler 17. A heatermeans 19 is connected to the frequency doubler 17 for altering thetemperature of the frequency doubler 17. Once the laser beam leaves themirror cavity 1, it passes through a semi-reflecting mirror 21 to siphonoff some of the laser beam 7.

In comparison with EP 0 329 442 A2, the process according to the presentinvention is based on measuring the noise of the laser itself and it byadjusting the doubler crystal temperature.

However, the usually disturbing laser noise, the low-frequency"flickering," can be used as a control input signal only poorly, becausethe amplitude modulation has relatively long period durations (typicallyin the second range and longer), whereas the modulation process proper,i.e., the drop in output, takes place abruptly (FIG. 2). Consequently,there is no possibility of preventing such a rapid, but relatively raredrop in power by countercontrol by a rapid reaction of the temperaturecontrol with the prior-art slow time constants.

However, our measurements revealed that the stable states with theabsence of flickering, in which the doubler temperature is consequentlyselected correctly, are correlated with an especially lowhigher-frequency laser noise. For example, FIG. 3 shows a typicalmeasured curve of the laser amplitude-noise frequency spectrum at asuitable doubler crystal temperature and absence of flickering (bottomcurve) and at poorly adjusted temperature with laser flickeringoccurring (top curve). It is clearly seen that the two curves have asignificantly different noise amplitude, especially in the frequencyrange between 10 and 500 kHz, whereas the noise below 10 kHz and above700 kHz is similarly intense for both temperatures and states of thelaser (with and without flickering). This noise component in theabove-described, significant frequency range shall therefore be used asa control signal for adjusting the temperature of the doubler crystaltemperature.

To do so, a small portion of the laser output radiation in the range ofa few mW or lower is tuned out of the useful laser beam 7 proper (thismay preferably be the frequency-doubled beam or, because it iscorrelated with this, also a part of the residual radiation of thefundamental wavelength, which leaks from the laser cavity 1) and isbrought to a photodetector 3 (e.g., a semiconductor diode). The electricoutput signal is then sent to an electric band pass filter 5 (optionallyafter amplification), so that only the noise signal in theabove-described, significant frequency range is then picked up at theoutput. The signal is subsequently rectified or squared in means 8. Thissignal can now be sent as a control signal to be minimized to atemperature control circuit 9, which will now actively adjust thedoubler crystal temperature to a minimal noise signal. Since the laseroutput can be stated as a fluctuating value or indefinitely only in somelaser arrays, a signal may be additionally derived already before theband pass filter, and this signal is integrated in another means 10 andis therefore proportional to the mean laser output, so that an indicatorof the power-related, relative noise is obtained by forming the quotientor ratio of this signal in division means 13 with the optionallylikewise rectified and integrated noise signal proper from the means 8behind the band pass filter. Depending on the quotient formation, thissignal may be minimized or maximized in max/min means 11, e.g., byderivation and by sending it to a temperature control unit 9. Accordingto another preferred embodiment, the noise signal is digitized eitherbefore or after the integration or squaring and is sent to amicroprocessor unit (e.g., of the type of 68 HC 11 or similar types),which already has suitable analog inputs and outputs; as an alternative,it is also possible to use processors with external analog-digital anddigital-analog converters; a digitally coded pulse width control signalmay also be sent instead of a digital-analog conversion at the output),which performs the integration or squaring digitally in the first casebut which sets the doubler crystal temperature by means of a suitablealgorithm in both cases such that a minimum of noise signal is received.The output signal is preferably sent directly to a temperature controlunit to change the doubler crystal temperature, or it is added in atemperature regulating unit as a change signal to the temperature setpoint. The control signal may, of course, also intervene at anothersuitable point of a temperature control unit. In the above-describedcase of fluctuating laser output, a low pass-filtered laser outputsignal may be subjected analogously to the noise signal to a quotientformation, as was described above, and then sent to the microprocessorunit in this case as well, or it may be sent via another input channeldirectly to the microprocessor unit, which will perform the quotientformation described digitally in this case.

An especially simplified arrangement is obtained by sending theoptionally amplified photodiode signal to a microprocessor, whichperforms both the band pass filtration and the necessary rectificationand squaring steps and the integrations and the quotient formation. Themicroprocessor may also assume the temperature control of the doublercrystal.

The process proposed thus makes it possible to actively trackfluctuation-free (flicker-free) states of the frequency-doubled laserradiation at a relatively small bandwidth and, unlike in the case of thechaotic controls, to obtain a purely linear control principle in orderto thus maintain the optimal doubler temperature even under changingenvironmental or disturbing effects and outside the thermal equilibrium.It is also possible to compensate (to a certain degree) aging andmaladjustment effects.

It may be advantageous to limit the range of control of the temperaturecontroller (with measures known from the state of the art) such that asecond minimum of the laser noise (a second optimal doubler temperature;the correct length of the crystal for its action as a quarter-wave platebeing periodic with λ/4) will not cause an ambiguity in the controlbehavior. Furthermore, too great a change in the doubler temperature mayalso lead, besides to ambiguities in the noise minimum, to an excessivechange in the laser output, which also calls for a limitation of therange of control. To prevent the control system from "hitting" theboundaries of this limitation, provisions may optionally be made here toensure that a jumping back into another temperature range located at asufficient distance from the limits of control will take place when thelimits of the limitation are reached. This may be carried out with easeby a corresponding programming of the microprocessor, especially in thecase of the digital design of the process described.

The features described in specification, drawings, abstract, and claims,can be used individually and in arbitrary combinations for practicingthe present invention.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

FIG. 1

KEY:

Resonatorspiegel=Cavity mirror

Lasermedium=Laser medium

Frequenzverdoppler=Frequency doubler

Heizung=Heater

teilreflektierender Spiegel=Semi-reflecting mirror

Laserstrahl=Laser beam

Temperature-Regelung=Temperature control unit

Differenzierer oder Minimier-Algorithmus=Differentiator or minimizingalgorithm

Division=Division

Gleichrichter=Rectifier

Tiefpaβ=Low-pass filter

(Integrator)=(Integrator)

Bandpaβ-Filter=Low-pass filter

Photodetektor=Photodetector

FIG. 2

FIG. 3

FIG. 4

What is claimed is:
 1. A process for minimizing the amplitude noise of asolid state laser with a frequency doubler crystal, the processcomprising the steps of:removing a portion of radiation from the solidstate laser; converting said portion of radiation into a portion signal;filtering said portion signal to remove frequencies outside of apredetermined frequency range and generate a filter signal, saidpredetermined frequency range including, frequencies having noiseamplitudes which vary dependent on a temperature of the frequencydoubler crystal; measuring said noise amplitudes of said filter signal;adjusting said temperature of the frequency doubler crystal to minimizesaid noise amplitudes.
 2. A process in accordance with claim 1,wherein:said converting includes sending said portion of radiation ofthe laser to a photodetector to create an electric output signal; saidfiltering includes sending said electric output signal of said thephotodetector to a band pass filter which filters out a frequency rangetypical of the noise to be minimized; said measuring includes one ofrectification, squaring and integration of said filter signal; saidadjusting of temperature is such that said filter signal from said bandpass filter and consequently an amplitude noise of the laser radiationare minimized.
 3. A process in accordance with claim 2, furthercomprising:amplifying said portion signal from said photodetector signaland sending it to a microprocessor which performs said filtering andsaid rectification or squaring steps and integrations as well asquotient formations.
 4. A process in accordance with claim 1, wherein:asolid laser material and the frequency doubler crystal are located in acommon cavity; said portion of radiation is one of a part offrequency-doubled laser output radiation and fundamental wave radiation;the solid laser is one of Nd:YAG, Nd:YAlO, Nd:YLF, Nd:YVO₃ ; saidfrequency doubling crystal is one of KTP, LBO, BBO, KNbO₃, LiNbO₃.
 5. Aprocess in accordance with claim 1, wherein:said predetermined frequencyrange has a lower limit between 10 to 100 kHz and an upper limit between300 to 700 kHz.
 6. A process in accordance with claim 1, wherein:saidpredetermined frequency range is of a magnitude and location to havesaid filter signal include a frequency range of substantially 100 kHz to300 kHz.
 7. A process in accordance with claim 1, furthercomprising:comparing said portion signal with said filter signal to forma ratio representing an indicator of power-related relative noise inpredetermined frequency range of said filtering; further adjusting saidtemperature of said frequency doubler crystal to one of maximize andminimize said ratio.
 8. A process in accordance with claim 7, furthercomprising:one of rectification, squaring, integration and low passfiltering of one of said filter signal before said comparing and saidportion signal in parallel with said filtering but before saidcomparing.
 9. A process in accordance with claim 1, wherein:an outputsignal, which is optionally subjected to low pass filtration and is setin relation to the signal behind the bandpass filter, is also picked upbefore the band pass filter, so that this ratio represents an indicatorof the power-related relative noise in the frequency range defined bythe band pass filter; a quotient represents a ratio of the noise signalbehind the band pass filter to the output signal before the band passfilter; said temperature of the frequency doubler crystal is set tominimized said quotient.
 10. A process in accordance with claim 1,wherein:said adjusting of said temperature is performed to prevent saidadjusting from locking in on a local minimum of said noise amplitude.11. A process in accordance with claim 1, wherein:said portion signaland said filter signal are modified to only include a single noiseamplitude minimum.
 12. A process in accordance with claim 1,wherein:said filter signal of is one of rectified, analogously squared,low pass filtered and integrated, said filter signal is then sent to ananalog input of a microprocessor, said microprocessor determines aminimum of said noise amplitudes numerically by means of an algorithm asa function of said temperature and adjusts said temperature via acontrol output such that said minimum of said noise amplitudes will bemaintained in said predetermined frequency range.
 13. A process inaccordance with claim 12, wherein:a signal, which is proportional to thelaser output, is additionally sent to the microprocessor at anotheranalog input, and the microprocessor determines a minimum of a quotientof the filter signal and the output signal or the maximum of thereciprocal values thereof numerically by means of a suitable algorithmas a function of the temperature and adjusts the temperature via acontrol output such that this minimum of the relative noise will bemaintained in said predetermined frequency range.
 14. A process inaccordance with claim 12, wherein:said microprocessor also performs saidadjusting of said temperature of the frequency doubler crystal.
 15. Aprocess in accordance with claim 1, wherein:said filter signal is sentto an analog input of a microprocessor, which numerically rectifies orsquares and optionally integrates this signal and determines the minimumof the noise numerically by means of a algorithm as a function of thetemperature and adjusts the temperature via a control output such thatthis minimum of the noise will be maintained in said predeterminedfrequency range.
 16. A process in accordance with claim 15, wherein:asignal, which is proportional to the laser output, is additionally sentto the microprocessor at another analog input, the microprocessor formsthe quotient of the filter signal and the output signal or itsreciprocal value and determines the minimum of this quotient of thefilter signal and output signal or the maximum of the reciprocal valuesthereof numerically by means of a algorithm as a function of thetemperature, and adjusts the temperature via a control output such thatthis minimum of the relative noise will be maintained in the saidpredetermined frequency range.
 17. A process in accordance with claim 1,wherein:said filter signal is one of rectified or squared and/orintegrated and a quotient of this filter signal and a signalproportional to the laser output or a reciprocal value thereof islikewise formed analogously, and that the output signal of the quotientformer is then sent to an analog input of a microprocessor, whichdetermines the minimum of the quotient of the noise and output or themaximum of the reciprocal values thereof numerically by means of aalgorithm as a function of the temperature and adjusts the temperaturevia a control output such that this minimum of the noise will bemaintained in said predetermined frequency range.
 18. A process inaccordance with claim 1, wherein:said adjusting of said temperatureincludes sending a temperature control signal is to a temperaturecontrol unit of the frequency doubler crystal or is added to a set pointof a temperature control unit of the frequency doubler crystal.